Acids and Bases

1. Acids and Bases

2. Acid/Base Reactions • Common examples of acid/base reactions are taking an antacid for an upset stomach, Using hair conditioner and spreading lime on soil • A study of acid/base reactions allows us understand what goes on in these everyday events

3. pH We can tell how acidic or basic a substance is by measuring something called pH

4. pH of household items

5. Some acids are dangerous and must be handled with care Eg. sulphuric acid • Others are harmless and can even form part of our diet Eg. Ethanoic acid in vinegar and citric acid in oranges and lemons

6. The word acid comes from the Latin word acidus meaning sour • Acids turn moist blue litmus paper red • Acids react with metals to release Hydrogen gas

7. Arrhenius’ Theory of Acids and Bases • Svante Arrhenius was a Swedish chemist who put forward a theory about acids and bases in 1884 • Picture here

8. He suggested that some substances dissociate into ions when added to water • Defined an acid as: An acid is a substance that dissociates in water to produce H+ ions

9. When Hydrogen chloride (HCl) is added to water it breaks up (dissociates) into H+ and Cl- ions HCl H+ + Cl-

10. The dissociation of Nitric acid and Sulphuric acid may be presented as HNO3 H++ NO3- And H2SO4 2H+ + SO42-

11. Acids like hydrochloric acid and Nitric acid which donate one H+ are called monobasic • Acids like Sulphuric acid which donate 2 H+ are called Dibasic • An acid like phosphoric acid H3PO4 which donates 3 H+ is called Tribasic

12. Strong and Weak acids • Acids such as Hydrochloric acid, Sulphuric acid and nitric acid are strong acids because they will fully dissociate in water – this means almost every molecule will break up to give H+ • Acids such as ethanoic acids are called weak acids as they do not fully dissociate in water

13. Hydronium ion • As chemists learned more about atomic structure it was realised that H+ could not exist on its own as this is just a single proton • What is formed in reality is a Hydronium ion H3O+ which occurs when the H+ joins to a water molecule

14. The formation of a Hydronium Ion • Fig 12.3

15. The hydronium ion is very common and is found in every acidic solution • It is more accurate to represent the dissociation of acids in water as HA H3O++ A- Example HCL H3O++ Cl-

16. Importance of water • If you add substances such as pure ethanoic acid or pure hydrogen chloride to solvents such as benzene or methylbenzene then they can’t form an acid they will only form an acid when mixed with water

17. Bases • A group of substances that behave oppositely to acids are called bases • They turn red litmus paper blue

18. Sodium hydroxide • NaOH is a common base found in the laboratory • Most bases have a soapy feel as they turn the oil on your skin to soap • Arrhenius defined a base as: A base is a substance that dissociates in water to produce OH- ions

19. The OH- ion is commonly called the hydroxide ion • When NaOH is added to water it dissociates or breaks up into Na+ and OH- ions NaOH Na+ and OH-

20. Strong and Weak bases • A strong base dissociates fully Eg. NaOH • A weak base doesn’t dissociate fully Eg. Calcium hydroxide Ca(OH)2 and Magnesium hydroxide Mg(OH)2 • NB A base that dissolves in water is called an alkali

21. Problems with Arrhenius’ theory • It is hydronium ions rather than bare H+ ions that exist • Definitions are only in aqueous solutions and don’t take account of solvents such as ammonia, benzene etc. • Not all acid base reactions require liquid eg Ammonia gas and hydrogen chloride gas react

22. Bronsted/Lowry Theory of Acids and Bases • 1923 Danish chemist Bronsted + English chemist Lowry independently proposed new definitions for an acid and a base • These simply state that: • A base is a proton donor • An acid is a protons aceptor

23. Consider This • When hydrogen chloride is added to water the following reaction occurs HCl + H2O H3O+ + Cl-

24. Consider This • When ammonia gas dissolves in water the following reaction occurs NH3 + H2O NH4+ + OH- Accepts a proton B/L base Donates a proton B/L acid

25. Water can act as either an acid or a base and for this reason is known as amphoteric or amphiprotic

26. Bronsted Lowry theory does not only refer to reactions where water is the solvent • In the following reaction between Hydrogen chloride and Ammonia HCl + NH3 NH4+ + Cl-

27. Advantages of the Bronsted/Lowry Theory over Arrhenius’ Theory • Arrhenius’ theory was confined to aqueous solutions • Bronsted/Lowry’s theory broadens the range of species that can be defined as acids and bases • Substances that are not classified as acids or bases under Arrhenius’ theory are classified as acids or bases in the Bronsted/Lowry theory

28. Note • An acid will only donate a proton when there is something there to accept it • A base will only accept a proton if there is something there to donate it

29. Conjugate acid/base pairs • Certain reactions are reversible and can happen in both directions • We use the symbol to show this

30. Consider This • If the following reaction is written as CH3COOH + H2O CH3COO- + H3O+ Ethanoic acid Water This means the reaction CH3COOH + H2O CH3COO- + H3O+ And the reaction CH3COO- + H3O+ CH3COOH + H2O Can take place

31. Identify the B/L acid in the following Equations • CH3COOH + H2O CH3COO- + H3O+ • CH3COO- + H3O+ CH3COOH + H2O

32. An acid changes to a conjugate base when it donates a proton • A base changes to a conjugate acid when it accepts a proton

33. CH3COO- is the conjugate base of CH3COOH • CH3COOH is the conjugate acid of CH3COO- Every base has a conjugate acid and every acid has a conjugate base Since CH3COO- and CH3COOH only differ by a proton we refer to them as a conjugate acid base pair

34. Why can H2O and H3O+ be called a conjugate acid base pair? • NB Study examples 12.1 and 12.2 on pages 140 and 141 of your text book

35. Neutralisation • When acids and bases react with each other in the right proportions they cancel each other out • When this happens both the acid and the base lose their characteristic properties • The solution formed is neutral and has no effect on litmus paper • This type of reaction is known as neutralisation

36. Neutralisation is the reaction between an acid and a base to form salt and water

37. Reaction between HCl and NaOH HCl + NaOH H2O + NaCl Acid + Base Water + Salt • The word salt is a general term used to describe the substance formed when the hydrogen in an acid is replaced by a metal or an ammonium ion

38. Understanding what happens • HCl + NaOH H2O + NaCl • In this equation the following ions are all present in solution H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O The spectator ions are crossed out as what is important is the reaction where the H+ (Or more correctly Hydronium ions )and OH- ions react to form water H + + OH- H2O

39. Everyday examples of Neutralisation • Medicine – Hydrochloric acid in stomachs helps digestion. Over eating and stress can produce too much acid. Antacids such as Alka Seltzer, Bisodol and Milk of Magnesia may be used to try to neutralise excess acid

40. 2. Agriculture – If soil is too acidic crop yields can be low, therefore farmers often spread lime (CaO) on soil to neutralise the acidity 3. Environmental Protection- In areas affected by acid rain limestone is often added to lakes to neutralise acidity

41. Toothpaste is slightly basic to neutralise acids that cause tooth decay • Baking soda may be used to neutralise the acid of bees • Vinegar is used to neutralise the alkaline sting of wasps • Shampoo is slightly basic and can cause scales on hair to stick out, conditioner is slightly acidic so neutralises the base flattening the scales and making hair more shiny and manageable